The present invention concerns magnetic motion sensors which generate output signals that have linear characteristics and which include a coil that surrounds a ferromagnetic core and at least one permanent magnet that can be moved in the longitudinal direction of the coil and which causes a magnetic saturation along a zone that is parallel to the ferromagnetic core.
Motion sensors are used, for example, for monitoring the piston travel in hydraulic or automatic systems. Sensors which function according to the magnetostrictive principle are known, but they have their disadvantages. They require extensive electronics to process the output, which renders the devices costly and not suitable for many lower cost applications. Such sensors further have mechanically complex sensing elements which are subject to mechanical stresses caused, for example, by vibrations and/or shock loads. Magnetostrictive sensors additionally consume relatively large amounts of electric current which makes it unfeasible to power such systems with batteries. Magnetostrictive sensors have an additional disadvantage, particularly when used to measure short measuring distances, because only a relatively small portion of the sensor can actually be used as an electric measurement length.
Further, magneto-inductive sensors are known. They employ a primary winding to generate a magnetic field and a secondary winding for a position-dependent field detection. These sensors have the disadvantage that a multitude of windings must be separately connected and operated, which can only be obtained at relatively high costs.
German patent publication DE 103 42 473 A1 describes a position sensor that has a ferromagnetic core surrounded by a coil with a winding density that varies in the longitudinal direction. It employs a movable magnet with which different sections of the core can be magnetically saturated so that the coil has a position-dependent characteristic. A disadvantage of this sensor is that, for many applications, the position signals must have linear characteristics. DE 103 42 473 A1 teaches that the winding density should result in a linear change in inductivity when the magnet is moved or repositioned by linearly increasing or decreasing the winding density over the measurement length. This significantly limits the largest possible measurement length. For example, when the winding density decreases linearly, areas or zones without an effective inductivity layer are present even when the measurement length is short. The measurement length might be increased (for example, to 400 mm) by arranging segments having constant winding densities next to each other instead of providing a continuous increase or decrease in the winding density. In this arrangement, the winding densities of the different segments would increase or decrease linearly. By maintaining the width of the segments small relative to the width of the magnet, for example 113, the significantly greater width of the saturation zone causes a smoothing of the stepped inductivity layer so that the overall characteristics of the output are smoothed to obtain a substantially linear characteristic. Such a construction increases the available measurement length, but has the disadvantage of an undesirable offset caused by the fact that the maximum inductivity reduction attained by saturation is small relative to the overall inductivity. Such an offset is undesirable and very costly to electrically compensate.
A further disadvantage of the sensor is that the measurement size, which is position-dependent, is the characteristic of the coil. However, many applications prefer that the position-dependent signal is a direct current or direct voltage value. As a result, the measured value must first be electronically converted into a direct current or direct voltage signal, which is costly.